Re: [PATCH v2 1/5] KVM: x86: Introduce KVM_REQ_RING_SOFT_FULL

[Peter Xu]

On Fri, Sep 16, 2022 at 12:51:31PM +0800, Gavin Shan wrote:
> This adds KVM_REQ_RING_SOFT_FULL, which is raised when the dirty
> ring of the specific VCPU becomes softly full in kvm_dirty_ring_push().
> The VCPU is enforced to exit when the request is raised and its
> dirty ring is softly full on its entrance.
> 
> Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c       | 5 +++--
>  include/linux/kvm_host.h | 1 +
>  virt/kvm/dirty_ring.c    | 4 ++++
>  3 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 43a6a7efc6ec..7f368f59f033 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10265,8 +10265,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	bool req_immediate_exit = false;
>  
>  	/* Forbid vmenter if vcpu dirty ring is soft-full */
> -	if (unlikely(vcpu->kvm->dirty_ring_size &&
> -		     kvm_dirty_ring_soft_full(&vcpu->dirty_ring))) {
> +	if (kvm_check_request(KVM_REQ_RING_SOFT_FULL, vcpu) &&
> +	    kvm_dirty_ring_soft_full(&vcpu->dirty_ring)) {
> +		kvm_make_request(KVM_REQ_RING_SOFT_FULL, vcpu);
>  		vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
>  		trace_kvm_dirty_ring_exit(vcpu);
>  		r = 0;

As commented previously - can we use kvm_test_request() instead? because we
don't want to unconditionally clear the bit.  Instead of making the request
again, we can clear request only if !full.

We can also safely move this into the block of below kvm_request_pending()
as Marc used to suggest.

To explicitly use kvm_clear_request(), we may need to be careful on the
memory barriers.  I'm wondering whether we should have moved
smp_mb__after_atomic() into kvm_clear_request() because kvm_clear_request()
is used outside kvm_check_request() and IIUC all the call sites should
better have that barrier too to be safe.

Side note: when I read the code around I also see some mis-use of clear
request where it can be omitted, e.g.:

		if (kvm_check_request(KVM_REQ_UNHALT, vcpu)) {
			kvm_clear_request(KVM_REQ_UNHALT, vcpu);
			vcpu->run->exit_reason = KVM_EXIT_IRQ_WINDOW_OPEN;
		}

Maybe it's a sign of bad naming, so we should renamed kvm_check_request()
to kvm_test_clear_request() too to show that clearing after that is not
needed?

Thanks,

-- 
Peter Xu